Flash evaporation of a liquid (single or multi-component) uses an ultrasonic nozzle to atomize the liquid into small droplets which are then dispersed in a container maintained at elevated temperature (evaporator). When the droplets contact the walls of the evaporator, they instantaneously transform to the vapor state without boiling. This process eliminates the separation or distillation of the individual components of the liquid and maintains the equivalent composition of the liquid in the vapor phase. The flash evaporation process is generally described, for example, in U.S. Pat. Nos. 4,954,371, and 4,842,893, which are incorporated herein by reference.
One problem with this process is that the atomization process is not perfectly constant, which results in fluctuations of the amount of liquid transformed into the vapor phase over time. In one application of the flash evaporation process, the vapor phase is transported to a nozzle where the vapor is directed towards a substrate that passes by the nozzle and onto which the vapor condenses to form a thin liquid layer. Fluctuations in the amount of vapor (as measured by the pressure in the evaporator) result in variation of the flux reaching the substrate, causing an undesirable variation in the thickness of the condensed liquid layer.
One solution to this problem was the introduction of baffles in the evaporator in an attempt to homogenize the vapor pressure before it exits through the nozzle. The use of baffles is described, for example, in U.S. Pat. No. 5,902,641, which is incorporated herein by reference. However, the use of internal baffles has been only partially successful in reducing the vapor fluctuation. Moreover, this method adds complexity and expense to the evaporator fabrication.
In addition, in order for a uniform liquid film to be formed on the substrate, the pressure in the evaporator should reach a steady-state condition in as short a time as possible. This reduces the overall time for the discrete coating process and increases process throughput for manufacture. One method that has been used to reduce the time needed to reach a steady state condition is to narrow the opening of the vapor nozzle through which the vapor exits to the substrate. Although this does reduce the time for the evaporator to reach a steady-state pressure, the machining tolerances of the vapor nozzle become more difficult to achieve, making it more expensive and difficult to fabricate components.
Therefore, there is a need for an evaporator having reduced variation in the amount of vapor reaching the substrate. There is also a need for a less costly evaporator which reaches steady state quickly.